Electroluminescent lamps, such as CCFLs, are used in a variety of applications, including illuminating liquid crystal displays, scanners, photocopiers and the like. The lamps themselves are small, relatively efficient and inexpensive. However, they must be driven by specialised driving circuits that are capable of providing an output voltage that is sufficiently high to ignite the lamp and sustain illumination of the lamp after ignition.
A block diagram for a conventional driving circuit 100 that is commonly used to drive a CCFL type electroluminescent lamp is depicted in FIG. 1. This driving circuit 100 includes an alternative current (AC) voltage source 102 and a transformer 104. The driving circuit 100 is shown here as connected to a CCFL 108 load.
In conventional driving circuits of the type illustrated in FIG. 1, the transformer 104 is typically connected to the AC voltage source 102 using a conventional push-pull type topology. In operation, the AC voltage source 102 alternately produces an input current in the primary windings 110, 112 of the transformer 104 so as to thereby generate an AC output voltage waveform 114 (in the form of a high voltage AC output waveform) at an output 116 of the driving circuit 100 for driving the electroluminescent lamp. As is illustrated, the generated. AC output voltage waveform 114 includes a positive half cycle 118 and a negative half cycle 120.
In conventional driving circuits of the type illustrated in FIG. 1, the direction of the winding current in each of the primary windings 110, 112 has a fixed direction. That is, the current alternately flows either from point A to points B and C, or otherwise, it alternately flows from points B and C to point A. Thus, the direction of these currents does not change during operation. Likewise, the relationship of the polarity and phase of the AC output voltage waveform to the primary windings will also not change; the positive half cycle will always be attributable to one of the primary windings, the negative half cycle will always be attributable to the other primary winding.
Although conventional driving circuits of the type shown in FIG. 1 operate satisfactorily, long term use may lead to a reduction in the usable life of the CCFL. Such a reduction tends to result as a consequence of the positive half cycle 118 and a negative half cycle 120 of the output voltage waveform 114 having a fixed relationship with the primary windings 110, 112. More specifically, and as a result of the above-described fixed relationship, if characteristics of the components used to supply the input voltage to the primary windings drift over time (or are not carefully matched), or indeed if the characteristics of the primary windings change, distortion of the output voltage waveform occurs. Such distortion typically results in an asymmetric AC output voltage waveform in which the peak magnitude of one half cycle is different (that is, greater or less than) to the peak magnitude of the other half cycle.
Asymmetry in the AC output voltage waveform tends to cause an uneven distribution of Mercury (Hg) within the CCFL in that the density of Hg at the end of the CCFL connected to the primary winding supplying the half cycle having the larger magnitude voltage will decrease over time. In a serious case, this will lead to blackening at one end of the CCFL tube and will adversely affect the performance of the devices using the CCFL (for example liquid crystal display, scanner, photocopier), even to the extent that such devices become unusable.
In view of the foregoing, it would be desirable to provide a driving circuit that solved the above-mentioned problems.